Once you know whether your main goal is hypertrophy (10-RM), strength/hypertrophy balance (5-RM), or strength and power (3-RM or 1-RM), knowing how much to lift is fairly simple.
Let’s assume you’ve chosen to train for a strength/hypertrophy balance (5×5, a basic beginner’s strength conditioning program). After testing your 5-RM, you should begin your 5×5 workouts using 80% of your 5-RM. For example, if you can strict press 100 lbs 5 times, then you will start with 5 sets of 5 reps using 80 lbs.
Using 80-90% of your RM is intense enough to encourage muscle growth and development, while at the same time is ‘easy’ enough to allow you to get enough training volume completed before exhaustion to maximize growth and at the same time maintain good form throughout the session. The closer the weight you use approaches your RM, the more likely it is your form will begin to falter, and training with bad form not only decreases the efficiency of your training (good form is designed for economy of motion, i.e. proper form should allow you to handle heavier weights) but is also unsafe (proper form is also designed to ensure that you are not placing poorly leveraged loads onto the body that increase the chance of injury).
Knowing when you should raise the weight can be tricky, but here are two simple approaches that you can follow, the first one being a little more conservative than the second one.
1) On your last work set, rep out (i.e. do as many reps as you can until you fail or until your form gives out. DO NOT continue lifting with bad form). If you can do 2 or more extra reps, you might want to raise the weight next time. If you can do 10-15 extra reps, you began too low and can raise the weight by quite a bit (you may want to test your RM again). If you can’t get 2 extra reps, stay at that weight until you can.
2) When you can complete every rep you set out to complete (e.g. all 5 sets of 5), try raising the weight next time. You may not make every rep in every set with the new weight, but just stay at the new weight until you can complete every rep in every set.
For upper body exercises, weight should be increased in 5 or 10 lb increments. For lower body exercises, 10 or 20 lb increments are more appropriate.
During the first few months of training, it is common to make quite large jumps in your RMs, due to the quick learning of the nervous system, including improved muscle coordination during the lifts and improved firing of muscle fibers. This is why you may underestimate your work sets during the initial stages of training, and may need to perform RM tests more frequently than you will later on once your nervous system plateaus. Once this happens, the amount of weight you can add each session will decrease drastically. Don’t get discouraged. This is normal, and it’s now time for the muscles to start adapting ahead of the nervous system. Muscle adaptation is slower than nervous system adaptation, and that is why your progression will begin to slow. Be mentally prepared for this stage, and have realistic goals set to get you over this hump.
Finally, RECORD EVERYTHING! Improvement doesn’t always come as an increase in weight. It may be that one week you make 20 out of 25 back squats at 225 lbs, and the next week you make 22. Don’t overlook those small gains, as they are signs that you are improving. Dedication and perseverance pay off, but you may miss it if you aren’t paying close enough attention.